For ideal multicolor printing, a set of inks must be selected in such a way as to be able to obtain the highest possible number of discernible color tones, each of which should be as free as possible of darkening effects.
All systems of printing inks, proposed up to now, are based on a set of three primary color inks, that is on the standardized primaries cyan, magenta, and yellow, as described in the German Standard DIN 16 538.
In the course of the practical application of such three color systems, however, a number of difficulties are apt to occur, i.e., some of the mixed color tones, obtained by overprinting, do not show brilliant color tones but appear dull and darkened as if they were overprinted with a gray ink.
As is well known, the degree of inking in multicolor printing can hardly be kept constant and when printing with the inks of the three-color-system, fluctuations in the amount of inking lead inevitably to clearly perceptive variations of the color tone or hue. It is particularly difficult to obtain a constant color balance when neutral tones must be printed which are composed of nearly similar percentages of all of the three primaries. It has heretofore been proposed to overcome these difficulties by the application of such a modification of the three color process whereby each one or at least two of the primaries are split into two inks, having the same color tone but different intensity. Also, there have been produced six-ink color prints, whereby besides yellow and black, a clearer and denser cyan and magenta have been applied each time. However, since the clearer and darker versions of these inks did not differ with respect to the spectral position of the density maximum, no gain in color effects could be expected.
The six-color-system represents only a very valuable version of the well-known three-color-system, which has proved itself to be advantageous, particularly in the case of printing processes wherein printing forms of the autotypical type are used.
In practical multicolor printing, it is usual to print, in addition to the three primary inks, a black ink from a fourth printing plate. In fact, the additional use of the black printer may contribute to a more constant performance of neutral tones but, on the other hand, it is evident that no improvement can be expected as to brilliant color rendition, particularly in the lighter color tones.
Also, it is well known in the printing practice to use one or more inks in addition to the standardized inks of the three-color system, for example in the printing of labels, packages, carton boxes, etc., so as to make certain that the special color tones, which are typical for the packaging of a definite product, appear permanently in a constant tone. Generally in the printing of packages, cartons, boxes, etc., the printer will not rely on the effects of overprinting primary inks but rather he prefers to use additional inks specially produced or selected to match the required color tone.
Such additional inks, however, do not have at all the character of primary inks of a three-color-system, since such inks are not selected under the viewpoint of the obtainability of a maximum number of color tones by overprinting.